This invention relates to a method of treating a sheathing tube of a spent fuel box of a fuel assembly of a nuclear reactor prior to disposal. The invention also relates to a rolling mill used in carrying out the treatment.
In a nuclear power reactor of the boiling or pressurized water reactor type, a reactor core is constructed from a large number of fuel assemblies, which at the bottom rest on a lower grid of supporting plates and at the top are fixed to a core grid. Each fuel assembly comprises a number of fuel rods, which are placed in a fuel box. The fuel box consists of a sheathing tube or fuel channel of, for example, a zirconium alloy, and a bottom piece at the lower end of the fuel channel. This bottom piece extends down into openings in the supporting plates. The bottom piece constitutes an inlet for cooling water and the sheathing tube constitutes a guide tube for control of the flow of cooling water around the fuel rods.
The fuel box is not consumed when in use in the nuclear reactor but it has a relatively limited life because deformation occurs due to relaxation, corrosion and ageing. Because the sheathing tube (or fuel channel) during use is subjected to intensive neutron radiation, ageing and brittling are accelerated and certain alloying materials included in the sheathing tube are converted into unstable isotopes which, when disintegrating, give off radioactive radiation for a long period of time. Further, radioactive products are deposited on the tube surfaces. These facts, together with relaxation, corrosion and ageing, mean that the reuse of the tube material involves a problem from the point of view of radiation and is technically unsuitable. Therefore, non-usable spent fuel boxes are deposited in waste plants.
Since fuel boxes have a relatively large volume, it is important to reduce their volume prior to disposal. One way to reduce the volume of the fuel box has been to remove the bottom piece therefrom and to thereafter divide the substantially square section sheathing tube diagonally to obtain two elongated, angle-shaped members. This division by splitting the sheathing tube in two along its length may, however give rise to chipping and scattering of fine radioactive particles. Also, during use the irradiation of the sheathing tube causes the tube material, particularly at the midportion of the tube, to become so brittle that the tube may break during the splitting operation, rendering its management or control very difficult. Packages of divided sheathing tubes are contained in casings prior to their disposal.
The present invention aims to provide a decrease in volume prior to the disposal of sheathing tubes of fuel boxes in a manner which is more favourable than hitherto used methods both from the point of view of handling, protection and economy. A considerable advantage of the invention is that in the event of rupture of the sheathing tube, the spreading of tube parts and fine-grained depositions on the tube surfaces can be completely prevented in a simple manner.